Rodrique:This is explained in the manual.
How can I configure Fedora GDM to only display a prompt with username and password field, instead of all the users in /etc/passwd?
http://library.gnome.org/admin/gdm/
Make sure to check the docs for the version of GDM you are using,
since the configuration mechanism changed in GDM 2.21 and later
versions.
Brian